Fixed displacement pumps used in many automatic transmissions regulate transmission line pressure. This is accomplished by a force balance on a regulator valve. A variable pressure control solenoid supplies oil pressure that acts on one side of the regulator valve. The fixed displacement pump supplies pressure that acts on the opposing side of the regulator valve. In order for the line pressure to remain constant or at a desired amount, the force balance must remain constant. Because speed of the fixed displacement pump increases with engine speed, the pressure supplied by the fixed displacement pump increases, upsetting the force balance.
To compensate for the increased pressure supplied by the fixed displacement pump, excess flow from the pump may be exhausted. However, increased exhaust flow may cause increased backpressure in the system. Excessive pressure to the internal  transmission components, such as the fixed displacement pump and the regulator valve, causes increased wear and premature failure of the components. Additionally, because line pressure becomes a function of engine speed, clutch fill times vary with engine speed. Variable clutch fill times may cause problems with shift control.